Nuclear medicine is one of the most rapidly developing fields of clinical investigation. The term deriving from its origin in nuclear physics involves administration by injection into a vein of a small dose of radioisotope (a radioactive substance emitting gamma rays). The bloodstream distributes the dose throughout the body and a suitably sensitive transducer records a history of this distribution.
Areas of the body having high "uptake" of the isotope or a rich blood supply show up as bright or highly illuminating sources while conversely those of low "uptake" or blood supply appear dark. In this manner any portion of the body or specific organ may be subjected to clinical investigation in a safe, reliable and noninvasive manner.
The most frequently employed nuclear investigation device is a radiation transducer having a scintillation crystal (i.e. one that emits light photons proportionately to received radiation energy quanta). The light photons are detected by a plurality of phototubes in close optical communication with the crystal generating electric signals indicative of the light's source and intensity. Cameras of this variety are generally referred to as an "Anger" or gamma ray cameras. U.S. Pat. No. 3,011,057, incorporated herein by reference, discloses such a device.
When exposed to a radioactive source, a scintillation camera of this type produces an image of the isotope distribution by recording the phototube outputs corresponding to the incidence of individual gamma rays on the crystal. The phototube outputs are interpreted and translated by electronic circuitry into orthogonal (X, Y) spatial coordinates and a third signal (Z) representative of the energy level of each gamma ray event. The energy Z signal is particularly useful for screening or filtering out unwanted detections which result from background radiation, scattering, etc. By establishing an energy window around the energy level known to be typical of the nuclide sought to be detected the desired X, Y and Z signals may be accepted for processing while unwanted signals are rejected.
A well known problem of gamma cameras relates to the inherent nonlinearities of camera design and construction. The nonlinearities, which are exacerbated with attempts to increase camera resolution, result in spatial distortion of image points. This distortion results in both nonlinearity and nonuniformity of image. In general, nonlinearity may be attributed to (X,Y) signal distortion. Nonuniformities can arise either from these same distortions or from variance in Z signal response as a function of (X, Y) source position. These spatially related inherent sources of image distortion may be corrected in various ways. U.S. Pat. No. 3,745,345, discloses one attempt to correct spacial non-linearities. However it has been found that the method disclosed therein creates further artifacts that distort the images. In U.S. Pat. Nos. 4,212,061 and 4,281,382 (which have a common assignee to the present application) X and Y correction factors are derived and stored for employment in correcting camera signals on-line while the image is being acquired.
As made clear by U.S. Pat. Nos. 4,212,061 and 4,281,382, incorporated herein by reference, the differences in Z signal, as a function of the source position, are of significance in correcting distortion due to nonuniformity of image. Accordingly, the above-identified patents disclose a procedure or method of normalizing the camera Z response as a function of source position. The method, which is described in detail in the patents, involves acquiring a separate energy histogram for each unique (X, Y) element of the camera image. The elements are defined by a 64.times.64 matrix in apparent space i.e. the uncorrected (X,Y) camera response. The histogram includes the number of counts and their associated energy level for each element. A standard search and fitting routine may be applied to the accumulated data to determine the peak, about which a low Z threshold value and a high Z threshold value are determined to define a window. These values are placed in a Z translation table having unique threshold pairs for each of the apparent space 64.times.64 matrix elements. The nonuniform response of the camera to gamma ray energy levels may then be compensated for according to which element the event originates in. By controlling how the Z threshold window is defined, one is able to not only maximize the detection of significant information but minimize recording of unwanted events.
The above-described method provides effective means for accepting, according to the apparent element of origination, only those (X,Y) signals having energies which will contribute to the construction of a meaningful picture or image; the present invention extends and improves that method by modifying how the energy selection or window is determined.